This invention relates generally to devices for protecting orchards of citrus trees from frost damage, and particularly to an apparatus and method for protecting the budunion of individual citrus trees from frost damage as well as other hazards.
The field of art surrounding frost protection of citrus orchards has become very well developed, although uniformly effective and cost efficient solutions have remained beyond the present grasp of the industry despite the magnitude of the problem. Damage due to frost, particularly when the trees are severely damaged or killed rather than merely affecting ripening fruit, can present both an immediate economic setback as well as devastating long term reductions in regional productivity. Catastrophic frost conditions in Texas during December of 1983, as well as four major freezes between 1981 and 1985 in Florida, have taken an estimated immediate toll of $2-3 billion in citrus trees in well over 300,000 acres, and those states have yet to regain the use of over 50% of that lost acreage.
Recurring frosts of that magnitude can be expected to occur sporadically every 1-2 decades, although minor seasonal frosts can damage several hundred acres at a time in an individual orchard at costs exceeding $10,000 per acre, and trouble spots such as low-lying ground or cold pockets can be repeatedly struck with frosts each year. Consequently, growers will often allocate up to $1,500 per acre purely for frost protection, at an average of 120-150 citrus trees per acre.
The many methods of frost protection can be roughly categorized, although various texts and publications have attempted to provide a more scientific cataloguing of the devices available to citrus growers and the relative merits and drawbacks of each. Some of the common considerations in evaluating frost protection devices and methods include material and labor costs, durability and reuse, adaptability, efficiency, effectiveness and operating ranges, state regulations, and detrimental side effects such as pollution or operating hazards.
Orchard heaters are a fundamental form of frost protection for all types of orchards. The heaters may comprise individual "smudge pots" which are placed surrounding the trees and produce radiant heat, or heat blowers or canopy heaters which warm and control the circulation of the air within an orchard.
Several types of oil-fueled heating devices are known, variously termed return stack, pipe line, jumbo cone, lemora, straight stack, lazy flame, and internal exchange heaters as examples. See, W. Reuther, The Citrus Industry Vol.III, U.Cal.Div.Ag.Sci. (1973) at pp. 338 et.seq., LCC-67-63041. One example of such an interconnected smudge pot system is disclosed in U.S. Pat. No. 3,470,863 to Payne. Solid fuel heaters are also known, varying in type from coal baskets to briquettes, wax candles, and reusable petroleum coke bricks. See Reuther, supra, at 359. A disclosure of an improved method for producing radiant heat from a two part flameless combustible oxidizing solid fuel source, as well as a discussion of the various heat sources listed above, may be found in U.S. Pat. No. 3,842,536 to Schick.
It is significant to note that the temperature increases discussed in the Schick '536 patent are in the range of 1.degree.-6.degree. F. during a period up to four hours after ignition. Maintaining citrus trees in frost conditions at or slightly below the freezing point of water (32.degree. F.), or alternately producing temperature increases of less than 10.degree. F., has generally been considered the criteria for a successful product. The USDA regards maintaining a temperature of 20.degree. F. for a period of four hours as satisfactory to qualify a product as a frost protective device.
More elaborate heating devices have also been devised, such as the combination heater and vapor generator disclosed in U.S. Pat. No. 3,964,465. Heaters, artificial fogs, microsprinklers, and air channeling devices, living and non-living windbreaks, chemical reagents and bioengineered frost preventative preparations have proven effective in many frost situations, but are subject to failure in winds or non-advective frosts and produce less than marginal guarantees of protection from abnormal or catastrophic frosts.
Heaters are effective when a forced temperature inversion can be maintained within an area of an orchard, but it is more frequently desired to reverse a natural temperature inversion and create turbulence in the air to mix and transport both cold and warm air, thereby helping to prevent sublimation on foliage or fruit. See Reuther, supra. Consequently, wind generating machines have been adopted particularly on the west coast to create turbulence or convection, and have been used in combinations with various types of heaters. Conversely, because foliage is a particularly poor heat conductor, another attempted procedure is to place a canopy over individual trees for retaining generated heat. Examples of such canopies may be seen in U.S. Pat. Nos. 3,830,014 to Baker and 3,706,160 to Delbert. These fabric canopies may be deployed using an articulated vehicle such as shown in U.S. Pat. No. 3,791,069 to Nelson, or a system of piping used for irrigation and pesticide application can also be used to produce a foam canopy such as disclosed in U.S. Pat. No. 3,563,461 to Cole.
More complex systems of heating areas of an orchard include the microwave system of U.S. Pat. No. 4,434,345 to Muscatell, an underground conduit system for delivering smudge, water, fertilizers and pesticides as disclosed in U.S. Pat. No. 3,354,579 to Gross, or the electrostatic heating of citrus trees as discussed by G.E. Horanic and G. Yelenosky in Proc.First.Int'l.Citrus.Symp., Vol. 2 (1969) p. 539 et.seq. See also Reuther, supra.
Such larger heat generating and environmental control systems are initially quite expensive to purchase and maintain, are labor and fuel intensive, are subject to state regulation, and still provide uncertain protection in many instances. The chief advantages are that larger systems can be used to produce widely varying BTUs depending on the needs of the regional climate, at the higher ranges a proportional increase in BTU production is relatively cost effective, the systems will generally operate for many years without replacement so that they may be amortized, and the expense of the systems may be reflected in decreased insurance premiums.
It has become more popular in recent years to provide for the protection of individual trees, particularly since research has shown substantial benefits in protecting the more fragile budunion (the graft junction between the scion which is necessary to continued propagation and the more resilient root stalk which conveys disease resistance and other benefits) during the early formative years of a tree's growth. Damage to the budunion can result in loss of the entire tree, and thereby the several years necessary to raise a tree to the same level of development from a seedling.
Many types of tree wraps have been utilized in the past, including plant products such as rice straw, corn stalks, palm leaves, and other insulating materials. Man-made insulative products including fabrics and artificial fibers, styrofoams and polystyrenes, thin plastic films or resinous sheets, asphalt-felt, and metallic foils have been among the products produced commercially. Examples of such individual packaging materials for small plants are disclosed in U.S. Pat. Nos. 4,646,467 to Morrisroe, 4,265,049 to Gorawitz, and 4,006,561 to Thoma. See also Reuther, supra. Banking soil or mineral compositions at the bottom of young trees to provide insulation and wind protection is also utilized by some growers. More resilient plastic sheeting or collars are known, such as the plastic tree band disclosed in U.S. Pat. No. 3,333,361, and the double-faced corrugated plastic material marketed by the South Bay Co. and the single faced two-tone corrugated protective wraps manufactured by the Diversi-Plast Co. of Minneapolis, Minnesota.
Small plants have been individually covered using devices termed "hot caps," representative examples of the various types of hot caps being shown in U.S. Pat. Nos. 1,747,967 to Bell, 2,665,523 to Hardman, and 4,018,003 to Mirecki. Such hot cap devices are of limited value for use in the case of trees, either being too small to accommodate a developed tree, too inflexible to be used with a rapidly growing tree, and not providing significant insulation against cold temperatures.
Some of the more effective developments in protecting citrus trees have involved conducting heat from an isolated source to individual trees. Examples of such systems are disclosed in U.S. Pat. No. 4,614,055 to Day wherein heated water is circulated throughout an orchard via a series of interconnected manifolds wrapped around the trunks of the individual trees, and U.S. Pat. No. 4,651,465 to Lilly in which tubing circulating heated water is positioned adjacent to the tree trunks within cylindrical thermal barriers surrounding the trunks and forming substantially sealed air spaces.
These systems can be utilized to keep the individual budunions warm, although the systems are complicated to install, require pumps for water circulation as well as heating units, are difficult to reposition as young trees develop, require replacing thermal barriers and caps, and are themselves subject to freezing, leaking, and other failures.
One experimental effort to obviate these problems has been to place a metal rod deep in the soil with the upper end wrapped against the tree trunk with foil-coated insulation. The metal rod extracts and conducts a small amount heat from the soil which is normally near 50.degree. F., thus providing an approximately temperature increase of 3.degree.-7.degree. F. at the trunk of the tree. While this system is relatively inexpensive and requires only minimal maintenance, the thermal protection which is provided is also of marginal value.
The protection of individual trees is also important to homeowners and gardeners as well as commercial growers. It is not uncommon for people to spend one or two hours covering a few small citrus trees in anticipation of cold weather or a frost, and homeowners are willing to devote far more time and expense to protecting their larger developed trees, since their cost of replacement is significantly higher than commercial growers, and the available methods of protecting individual trees are substantially limited. Another recent and very popular development is the Reese Wrap disclosed in U.S. Pat. No. 4,341,039 to Reese and discussed by Yelenosky in HortScience, Vol. 16(1) p. 44 et.seq. (February 1981) and Proc.Fla.S.Hort.Soc. 92:25-27 (1979). The Reese Wrap comprises an outer cover molded from an insulating material such as polystyrene enclosing a pack of water filled tubes which surround the tree trunk. In theory, the approximately 0.5 liters of water contained within the water pack will release 36 Kcal. of heat when freezing at 32.degree. F. due to the molar heat of fusion of water.
The stability or cold resistance of citrus trees varies depending upon the type of tree, its planting condition, and the type of frost. In general, temperatures of 32.degree. F. and below are required in order to produce clinging frost or direct damage to the tree due to internal freezing, however the safe temperature margin for protecting orange and grapefruit trees can often be very narrow. Most varieties of developed orange trees can withstand temperatures of 28.degree.-29.degree. F. for an indefinite period, while temperatures of 25.degree.-26.degree. F. can cause severe damage within 3-6 hours and complete loss in 8 hours. Young seedlings or developing trees are far more fragile, and may be subject to complete loss at 32.degree. F. Moreover, it is often difficult to calculate the virtual temperature at the bark surface since moisture, ground and wind conditions may have a substantial influence.
In practice, the Reese Wrap has not proven completely effective, particularly in situations of extreme cold and rapid temperature drops. The addition of silver iodide or phenazine to the water as an ice seeding catalyst has proven helpful in combating the problem of supercooling during a strong freeze (although the increase in molarity causes a proportional decrease the the actual freezing temperature of the water) but the margin for error at temperatures near or slightly below freezing is extremely small for many tree varieties. Moreover, the Reese '039 patent suggests that a typical device could only produce sufficient heat to prevent tree damage for 13 hours, while Yelenosky, supra, states that 20 hours of protection can be afforded at non-critical temperatures. Once the water is frozen, however, the ability of this product to further protect the tree is reduced dramatically, and damage may be caused by pressure from the frozen water if the wrap is misapplied, not removed promptly, leaks, or acts as a catalyst to freezing or sublimation of the surrounding water vapor. If the temperature of the wrap rises very gradually, particularly due to increased sunshine rather than increased air temperatures, the frozen water can actually lower the temperature of the tree as the water reabsorbs the same quantity of heat of sublimation upon melting, thereby increasing the time the tree is subject to damage. The frozen wrap can be difficult to adjust or remove in order to prevent these problems. Finally, the Reese Wrap is very difficult to use in connection with larger or developed trees, since the molded insulators limit the application of the wrap to larger trunks and branches.
The use of water both as an insulator from the surrounding cold air and as an exothermic heat source due to the release of the thermal energy of fusion during freezing was known prior to the Reese '039 patent, representative examples of similar applications being shown in U.S. Pat. Nos. 4,267,665 and 4,137,667 to Wallace. Water filled jackets or covers are more effective in the case of small plants which do not transfer and dissipate heat upwardly as quickly as trees, which can be fully enclosed so as not to be subject to advective air conditions, and which do not require contact between the plant and water filled jacket.